Method and device for zinc electrowinning from sulfate solutions

ABSTRACT

The invention relates to a method and a device for electrowinning of zinc from sulfate solutions. The electrolyzer for zinc extraction according to the invention includes a bath-reactor which is equipped with power supply buses, a pipeline of two pipes for supplying the electrolyte, cathodes, anodes, plastic strips along the height of the anodes, an overflow for electrolyte discharge by gravity. The cathodes have an area of 3.5-10 m 2 . The height of the electrolyzer is at least two times greater than its width, and the length of the electrolyzer depends on the number of the electrodes. The bottom of the electrolyzer is divided by a groove along the length of the bath.

FIELD OF THE INVENTION

The invention is related to a method and a device for zinc electrowinning from sulfate solutions.

BACKGROUND OF THE INVENTION

Various methods and devices are known for obtaining electrolytic zinc and 90% of its production is realized by hydro-electrometallurgy. In the conventional electrolytic baths the process is carried out with current densities of ˜400÷600 A/m², cathode surface ˜1.0÷1.6 m² and manual stripping of zinc cathodes from the cathode sheets in 24 hours. A novelty in this direction, the application of which began approximately 25 years ago, are the so called Jumbo baths and electrodes of an area of ˜2.60÷3.40 m², each bath charged with ˜80÷120 cathodes. This technology is now being used by the modern electrolytic zinc plants and zinc stripping is done mechanically by various devices in every 48 hours at ˜3100÷3500 kWh/t energy consumption.

The disadvantage of all known processes and apparatus, regardless of the relative increase in productivity, is the main factor of effectiveness in electrowinning of metals, namely electricity consumption. The latter represents ˜70% of the total processing costs for zinc extraction, along with those for electrode management, electrolyte cooling, labor costs, consumables, etc.

TECHNICAL DESCRIPTION OF THE INVENTION

The essence of the method and the device according to the invention or the new technology and equipment for zinc electrowinning is the creation of a new generation of electrolyzers regarded as a kind of rectors. This is dictated by the process which is a complex function of electric potentials, oxi-reduction acts and hydro-aerodynamic environment in which the bath (reactor) is conditionally divided into upper and lower and, respectively, left and right symmetric zones. This design and, respectively, the process are consistent with the electrode potentials, productivity, electrode configuration, fixation and positioning of the cathodes and anodes, their formation and processing, the electrolyte circulation system and other factors. Besides, the separated gases—oxygen on the anodes and hydrogen on the cathodes—create a peculiar “fluid bed” and contribute to the movement of the electrolyte between the electrodes from the bottom to the top and vice versa apart of them. Furthermore, the charging of the fresh solution at the pressure of ˜1.5÷2.0 atm., its leading away by means of an overflow as a waste solution and the anode fixing devices have favorable effect on the movement of the solution (FIG. 2, [a] [b]).

In principle the electrolyzer can be formed longitudinally by two, connected by an overflow, baths, charged by a common circulation line, so that the velocity of the circulation doubles and it is possible to place additional cathodes and anodes (FIG. 5, [a] [b]).

The aim of the method and the device according to the present invention of a new generation of zinc electrowinning is, while maintaining high productivity, to increase process efficiency by reducing energy costs by ˜20÷30% and other related processing costs as a whole at comparable investment costs for construction, machinery, equipment and installation (engineering).

This is achieved by the introduction of a new design of an electrolysis bath and, respectively, electrodes, loaded into it compared to the existing ones so far.

What is characteristic here is that, while comparatively maintaining the accepted in practice width and length of the electrolyzer, its height is doubled, i.e. the mentioned upper and lower zones are already formed (FIG. 1). This allows increasing the effective area of the electrodes—min. 3.5 m² and max. ˜10 M ² for one cathode; this change is consistent with the features of the process, its parameters, etc., “dispersion capacity of electrolyte”. In this way the used current density of up to 500 A/m2 can be reduced to 250÷300 A/m2, leading to a reduction in the electrode potentials, energy consumption, respectively, without affecting the high performance at periodic deposition time of 48 to 96 hours. This change in the bath and electrodes configuration determines the changes in other parameters, which is indicated in the attached figures.

FIG. 1 shows the overall appearance of the device, and FIG. 2 [a]—its projections, representing the bath-reactor 5, equipped with power supply buses 6, pipeline 1 for feeding the electrolyte, loaded with cathodes 3 and anodes 4, which are necessary for the process. Electrowinning is realized by feeding the process solution through pipeline 1 with the pressure of ˜1.5÷2 atm. and electric power—by buses 6. The electrolyte enters the bath through two pipes 1 by gravity at the upper end and the middle of the bath (FIG. 2 a) and leaves the reactor at the other end by means of an overflow 2 along its entire width (FIG. 2 b). The fed electric current is distributed between the electrodes—cathodes 3 (FIG. 2) and anodes 4 (FIG. 2) and the process of electrowinning is carried out with the current density of ˜250÷500 A/m². The electrolyte temperature at the entrance of the bath is ˜25÷30° C. and, respectively, ˜36÷38° C. at the outlet; sulfuric acid content is ˜120 to 200 g/l. The speed of electrolyte circulation is maintained at ˜1.5÷3 bath volumes per hour and the distance between the electrodes is 80÷86 mm.

The used anodes 4 (FIG. 3 a and FIG. 3 b) are made of lead, containing ˜0.5÷1.0% Ag (cast of rolled) and are formed by two symmetrical sheets, copper bar 7 for upholding in the bath and conducting the current and plastic strips 8 for fixation (linear speed ˜0.15÷0.30 m/min).

FIG. 4 shows the plastic strip in two projections. The strip is secured along both ends of the anodes and helps their accurate positioning in the bath. Besides, the anodes are formed in such a way that their effective area increases with the height thanks to their vertically riffled surface (FIG. 3 b); the anodes are ˜12÷18 mm thick.

A version of the offered device, according to the present invention, is the twin electrolyzer, or two baths connected in a common system (FIG. 5 a and FIG. 5 b). In this case the electrolyte enters the baths by means of two pipes 1 at their upper ends, or the total number of the pipes is 4 (FIG. 5); power is fed by separate buses for the two volumes of the twin bath where the spent solution leaves by gravity the first bath along its entire width at the top and by means of a siphon-overflow enters the second bath bottom—up (pos. 9, FIG. 5).

The bottom of the electrolyzer (FIG. 2 a) is divided along the length of the bath by groove 11 which allows the settling of the electrode sludge (relative weight ˜5 g/cm³) and its periodical removal, while the anode surfaces are cleaned outside the bath by means of a suitable treatment (including ferrous sulfate solution) and collecting the sludge (above 60% MnO₂) to use it as an oxidizer. Anode cleaning is carried out in ferrous sulfate solution followed by treatment with rotating brushes and water shower under pressure; anode sheets are corrected only if necessary by means of an appropriate “pressing”.

DESCRIPTION OF THE ENCLOSED FIGURES

FIG. 1—General view of the device of the present invention;

FIG. 2 a—The device of the present invention, shown in three projections;

FIG. 2 b—Electrolyte overflow of the present invention;

FIGS. 3 a and 3 b—Lead anode for zinc electrowinning of the present invention;

FIG. 4—Plastic (PVC, PPL, etc.) strip-clamp for fixing the electrodes of the present invention;

FIG. 5 a and 5 b—Another version of a twin bath of the present invention.

EXAMPLES OF PREFERRED EMBODIMENTS Example 1

Electrowinning is realized in an electrolyzer of the volume of ˜30 m³, height—3.5 m and width—1.6 m, fed with 60 cathodes 3, each one having an area of ˜6 m². The density of the supplied current is ˜300 A/m². The electrolyte enters the bath-reactor 5 through two (diffusion) pipes 1 at the speed of ˜500÷700 l/min and exits along its entire short side through overflow 2. The lead anodes 4 are formed by two symmetrical parts and their surface is shaped like stepped grooves with tapered lower part and at least 80% of anode 4 is fixed by plastic strips, which are supplemented by plastic washers 10 at the middle and lower end of anode 4. The mixed electrolyte enters the reactor, containing 70 g/l zinc and 180 g/l sulfuric acid at the temperature of ˜30° C. (˜37° C. at the outlet) with the necessary additives; yield is 90% and energy consumption—˜2600 Kwh/t zinc.

The duration of the cathode cycle is ˜72 h; metal purity is 99.995% Zn.

Example 2

The process is carried out in a twin electrolyzer, formed by two baths with an intermediate wall, connected by a siphon, along their entire width. The baths are charged with 62 cathodes each, i.e. ˜124 cathodes with an area of ˜6 m² each; circulation is achieved with 4 pipes—diffusers (two at each inlet) in their upper parts and in the middle, overflowing between them top-down at the speed of ˜800÷1200 l/min. Anodes configuration and fixing elements are the same as in Example 1. Current density is ˜300 A/m² and the cycle of zinc deposition is 72 h. The yield is ˜90%, energy consumption—˜2700 Kwh, purity ˜99.995% Zn with the same electrolyte. Effective sludge removal is done in ˜30 days. 

1. An electrolyzer for zinc electrowinning, characterized in that it includes a bath-reactor (5), equipped with power supplying buses (6), a pipeline of two pipes (1) for feeding the electrolyte, cathodes (3), anodes (4), plastic strips (8) along the height of the anodes (4), an overflow (2) for electrolyte exit by gravity, where the cathodes (3) have an area of 3.5-10 m² and the height of the electrolyzer is at least two times greater than its width. The length of the electrolyzer depends on the number of the electrodes, and the bottom of the electrolyzer is divided by a groove (11) along the length of the bath (5).
 2. An electrolyzer according to claim 1, characterized in that the anodes (4) are made of pure lead, alloyed with 0.3 to 1.0% Ag.
 3. An electrolyzer according to claim 1, characterized in that the height of each anode (4) is divided into two equal parts and their total effective area is greater than that of the cathode (3), and the thickness of the anode (4) is 12-18 mm.
 4. An electrolyzer according claim 1, characterized in that power feeding buses (6) are made of copper and there is a copper bar (7) fixed on them.
 5. An electrolyzer according claim 1, characterized in that the plastic strips (8) are installed so as to cover the edges of anode (4) to at least 80% of the height and in the middle and the lower part the anodes are fixed by a plastic disc washer (10) between anode (4) and cathode (3).
 6. An electrolyzer for zinc electrowinning, characterized in that it consists of two connected electrolyzers according to claim 1, where between the two baths is provided a siphon-overflow (9) for the solution, leaving the first bath by gravity and entering the second bath from the bottom to the top.
 7. A method for zinc electrowinning with a device according to claim 1, characterized in that it consists of: feeding process electrolyte solution, containing zinc, sulfuric acid ˜120-200 g/l and additives as an option at the temperatures of 25-32° C. at the inlet and 36-38° C. at the outlet, and pressure 1.5-2 atm., through a pipeline (1) into the bath-reactor (5) and electric power by means of the buses (6); electrolysis at the current density of 250-500 A/m², maintaining electrolytic solution circulation speed of 1.5÷3 bath volumes per hour, where the anode-to-anode distance is ˜80-86 mm and the duration of the cathode cycle is 72 hours (48-96 hours), and the temperature of the electrolyte at the exit is 36-38° C.
 8. A method according to claim 7, characterized in that the process electrolytic solution contains 120-200 g/l sulfuric acid. 